Power amplifiers are commonly used as general purpose transmitters for wireless base-stations and handsets. In these applications, the power amplifier serves to change the power of an incoming signal to an output power suitable for transmitting the signal over a communication medium, such as the electromagnetic frequency spectrum in the case of a modulated RF carrier.
The power of a signal is directly proportional to the amplitude or voltage level of the signal (P=V.sup.2 /R) and is typically expressed in dBm units, which represent the logarithmic power of the signal with respect to 1 mW, (P in dBm units=20 log.sub.10 (V/1.sup.-3)). The change in power of a power amplifier is expressed in decibel (dB) units, (P change in dB units=20 log.sub.10 (V.sub.out /V.sub.in)). For example, a +6 dB change in power represents a gain in power by a factor of 2; a +20 dB change in power represents a gain in power by a factor of 10; and a +40 dB change in power represents a gain in power by a factor of 20. Negative dB changes in power represent reductions in power by an inverse factor. For example, -6 dB, -20 dB, and -40 dB, represent reductions in power by factors of 1/2, 1/10, and 1/20, respectively.
To maintain a desired output power over time, the power of the outgoing signal is continuously monitored and adjusted by a feedback circuit operably coupled between the output and gain control input of the power amplifier. The feedback circuit increases or decreases the gain of the power amplifier, and thereby the power of the signal, by an amount corresponding to the change in power needed to correct for fluctuations in the output power. These fluctuations are normally caused by environmental changes, such as changes in temperature, or spurious changes in the power of the incoming signal.
In more detail, the amplitude of the outgoing signal is first rectified and integrated by a power detector and loop filter in the feedback circuit to obtain a D.C. voltage which corresponds to the actual output power of the signal. The D.C. voltage is then compared by a comparator with a reference D.C. voltage which corresponds to the desired output power. The comparator outputs a D.C voltage to the gain control input of the power amplifier which corresponds to the difference between the output power D.C. voltage and the reference D.C. voltage. This feedback signal from the comparator serves to increase or decrease the gain of the power amplifier to correct fluctuations in the output power from its desired level, and thereby maintain the power of the outgoing signal at the desired power level.
Beyond maintaining a desired output power over time, existing transmitter applications also require that the output power be changed to different, discrete levels over varying ranges in power. One way to change the output power is to change the reference D.C. voltage from one which corresponds to the present output power to one which corresponds to the new, desired output power. In this way, the difference between the D.C. voltage and the reference D.C. voltage will correspond to the increase or decrease in gain required to change the output power of the outgoing signal to the new desired output power.
This method is problematic for transmitter applications having wide dynamic ranges because of the small dynamic range of the typical, off-the-shelf power detector, which is often on the order of 10 dB. The dynamic range of a power amplifier or power detector is the maximum dB range in power which the amplifier or detector can operate over without unacceptable levels of error.
In other words, because the voltage-current characteristic curve associated with the typical power detector is non-linear, the output power sensed by the power detector cannot change by more than 10 dB without unacceptable levels of error, whereby the power detector produces a D.C. voltage which does not correspond to the actual output power--for example, it does not correspond to within 1 dbm as required by the IF-54 standard for base station transmitters. Transmitter applications, on the other hand, often require that the output power change by more than 10 dB, in other words, over a wide dynamic range.
Briefly, by way of example, to change the output power from 10 dBm to 25 dBm, a 15 dB change, the reference D.C. voltage input to the comparator is changed from a 10 dBm voltage to a 25 dBm voltage. Assuming that the power detector has a 2 dBm error outside its 10 dB dynamic range, the power detector will only output a 23 dBm D.C. voltage to the feedback input of the comparator. To maintain equality between its two inputs, the comparator will increase the gain of the power amplifier and thus the output power by another 2 dBm, to 27 dBm, so that a 25 dBm D.C. voltage (the 27 dBm output power minus the 2 dBm error) is input to the comparator. This leaves the output of the power amplifier at the wrong power level of 27 dBm.
One way to compensate for the error associated with the power detector outside its dynamic range is to adjust or calibrate the reference voltage. For the above example, the reference voltage would be adjusted or calibrated to a 23 dBm voltage, rather than a 25 dBm voltage, when a 25 dBm output power is desired. Since the error associated with the power detector is not fixed, in order to make these adjustments or calibrations for each of the different power levels over a wide dynamic range, a number of additional components must be added to the feedback circuit, increasing the complexity and cost of the power amplifier, and decreasing its reliability.